What is the opposite gender of a alumna?

Alumna = a woman who is a graduate or former student of a specific school, college, or university.

Alumnae = more than one woman who is a graduate or former student of a specific school, college, or university.

So, the opposite gender* to an Alumna is an Alumnus.

Alumnus = a male graduate or former student of a specific school, college, or university.

Alumni = more than one male, or any combination of male and female graduates or former students of a specific school, college, or university.

*Note: The term "opposite" is misapplied when referring to Latin genders since there are three genders: Male, Female, Neuter. The opposite of "female" is "not female".